The present invention relates, in general, to temperature controlled equipment techniques in chemical reactors and, in particular, to a new and useful temperature control system for an olefin oxidation reactor which regulates the rate of coolant flow to maintain the reactor temperature within a desired temperature range, during the operation of the reactor as well as during start-up, shut-down and transient operating conditions.
Various techniques and systems are known for controlling chemical reactors.
In addition to the non-anticipating but relevant patents set forth in the parent application, U.S. Pat. No. 3,471,582 to Lupfer discloses an arrangement wherein a desired temperature across an exothermic reactor is maintained by controlling the reactor feed temperature in response to a difference in the temperature between the reactant feed and product stream from the reactor until a maximum predetermined product temperature is obtained.
U.S. Pat. No. 3,271,472 to Ogle et al discloses apparatus for controlling the operation of a thermal cracking furnace. Since the thermal cracking of hydrocarbons is an endothermic reaction, it is necessary to maintain a maximum possible temperature within the equipment limits. A minimum temperature is not considered or important in Ogle et al.
Also see U.S. Pat. No. 4,249,907 to Callegas which discloses a selective hydrogeneration process wherein at least one catalyst is utilized. The temperature of the feed steam to the catalyst bed is controlled so as to maintain a desired reaction temperature in the catalyst bed.
In an olefin, in particular ethylene, oxide manufacturing process, ethylene and oxygen or air is mixed and fet to an isothermal multitubular reactor. Ethylene is oxidized into ethylene oxide in the presence of a catalyst and carbon dioxide and water are produced as by-products. Reactor temperature control objectives are:
Operation at the most economical temperature; PA0 Operation within a safe zone; PA0 Maximum conversion to ethylene oxide while minimizing by-products; PA0 Reduction consumption of coolant; PA0 Avoidance or elimination of unsafe operation; and PA0 Reduced operator attention. PA0 1. The most economical temperature for oxidation is one at which the highest conversion to ethylene oxide occurs rather than to by-products. PA0 2. Catalyst selectivity increases as the reaction temperature is lowered while ethylene conversion increases with increasing reactor temperature. Thus, temperature requirements for high selectivity and high conversion are opposed. This results in a narrow temperature range for reactor operation. PA0 3. Increase in reaction temperature produces two effects: (1) overall rate of ethylene oxidation increases, and (2) catalyst selectivity to ethylene oxide decreases such that relatively more ethylene is converted into carbon dioxide and water. Moreover, heat generation increases by the fact that more ethylene is oxidized and overall reaction becomes less selective. Consequently, increase in temperature may result in:
Reactor temperature control is of key significance because of the following factors:
a reactor runaway condition; PA1 catalyst poisoning; PA1 increased coolant demand; PA1 an unsafe operating situation; and/or PA1 increased operator attention.
Hence, neither a temperature rise nor a temperature drop is desirable.
In the state of the art system, reactor temperature control system is based on manipulating coolant flow rate. Its set point is directly based upon average reactor temperature. These control schemes result in almost all the deficiencies described above.